The present invention relates to a heat-sensitive recording sheet, and more particularly, to such a heat-sensitive recording sheet having excellent color-forming properties, other recording aptitudes and stability of background brightness.
A heat-sensitive recording sheet that utilizes a heat color-forming reaction occurring between a colorless or pale-colored chromogenic dyestuff and a phenolic material, or an organic acid is disclosed, for example, in the Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and in the Japanese Laid-Open patent application No. 27736/1973, and is now widely applied for practical use.
In general, a heat-sensitive recording sheet is produced by applying on the paper surface the coating which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and a color-developing material, such as a phenolic substance, into fine particles, mixing the resultant dispersions with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. When this sheet is heated, the coating undergoes instantaneously a chemical reaction which forms a color. In this case various bright colors can be advantageously formed depending upon selection of specific colorless chromogenic dyestuff.
These heat-sensitive recording sheets have now been finding a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, automatic ticket vending machines, and so on.
Such recording equipment has heating elements such as a thermal head or heating pen (stylus). When heated on contact with heating elements or by energy of predetermined light, the heat-sensitive recording sheet forms a color for recording.
A thermal head is provided with minute resistors on a substrate and is ordinarily subjected to its heating and cooling cycles repeatedly at a short cyclic period of 0.5-20 milliseconds by applying an electric current, and the color-forming layer of a heat-sensitive recording paper on contact with the thermal head is melted with the heat energy owing thereto and causes a color-forming reaction for recording.
Thermal heads that have been produced so far are diverse in their materials used and configuration. Accordingly, since the requirements for heat-sensitive recording sheets vary largely with the performance, controlling method, recording conditions (impressed voltage, pulse width, surface temperature, contacting pressure, recording speed and contents of information to be recorded) or the like of the specific thermal heads used, the matching between the respective heat-sensitive recording sheets and the recording equipment is very important. Especially in recent years, as the applications of recording equipment tend to be diversified and require a higher performance, a higher quality has come to be required for heat-sensitive recording sheets to be used thereon.
In printers, e.g. a dot matrix of 5.times.7 array is previously sufficient to express the alphabet and numerals, but a dot matrix of 16.times.18 array or 32.times.32 array is now required for the exact expression of Chinese characters.
In facsimiles which use thermal heads brought into line, a density of circa 4 lines per mm was previously sufficient, but a density of 8 or more lines per mm is now required for the improvement of image, that is, improvement of resolution.
Normal recording velocity of printers which was previously 30-60 characters per second, has become faster and faster, and is nowadays 120 or more characters per second.
Facsimile equipment requires, nowadays, less than one minute for recording a sheet of A4-size (210 mm.times.297 mm), although, previously, several minutes for recording was required.
For such high speed recording, it is required to shorten heating time on applying an electric current, that is, to shorten a pulse width, which causes a reduced heat energy. In this case, it cannot be expected, due to consideration of durability of the thermal heads, to raise the temperature of the thermal heads by raising impressed voltage as a method for compensation of the reduced heat energy.
Also, the heat energy of the thermal heads in the recording equipment capable of such high density and such high speed is more minimized. Therefore, it is required that the heat-sensitive recording sheet has a higher color-forming sensitivity sufficient for producing clear chromogenic records with such small heat input from the thermal head.
Therefore, a colorless dyestuff or an acidic material, such as organic acids and phenols, in the color-forming layer must be melted with minimized heat energy to cause the color-forming reaction. If possible, it is desirable to cause the color-forming at a temperature of 70.degree. C. to 120.degree. C.
The colorless chromogenic dyestuffs for thermal recording sheets which have the structure of lactones, lactams, spiropyrans etc. possess usually a melting point of 160.degree. C. to 240.degree. C., but there are no stable dyestuffs that are melted at the aforementioned low temperature.
On the other hand, there are many color-developing agents, such as organic acids and phenolic substances, which were used in combination with a colorless chromogenic dyestuff and which were disclosed in the Japanese Patent Publication No. 14039/1970 and various literatures. Particularly, 4,4'-isopropylidendiphenol (bisphenol A, having a melting point: 156.degree. C. to 158.degree. C.) is a useful, desirable phenolic substance and is now widely put into practical use, since it is stable, inexpensive and readily available. But 4,4'-isopropylidendiphenol has as its disadvantage a high heat color-forming temperature.
Then, phenolic substances of low melting point, for example, monomeric phenols such as 4-tertiary-butylphenol (m.p. 94.degree.-99.degree. C.), .alpha.-naphthol (m.p. 95.degree.-96.degree. C.), .beta.-naphthol (m.p. 119.degree.-122.degree. C.) etc., deteriorate preservability and stability of a heat-sensitive recording sheet, which is gradually colored at room temperature and has a phenolic odor. Therefore, such phenolic substances are not acceptable for practical use. The Japanese Patent Publication No. 12819/1979 discloses that p,p'-(1-methyl-normalhexylidene-) diphenol has a low melting point (m.p. 99.degree.-103.degree. C.) and gives a heat-sensitive recording paper with excellent stability and color-forming property, but there is a shortcoming that such substance has difficulty in synthesis and is not readily available.
As described hereinabove, there are almost no compounds having effective melting points for high speed and high resolution recording among colorless chromogenic dyestuffs and organic acids, such as phenols, which are possible to be used practically.
The color-forming temperature of heat-sensitive recording sheets depends upon the melting of either one of color-forming materials composed of a chromogenic dyestuff and an acidic material, such as organic acids and phenols.
Where both color-forming materials have a higher melting point, a substance with a lower melting point is added thereto. In this case, if one of the color-forming materials is dissolved by melting this substance, it is possible to cause a color-forming reaction even at a lower temperature. Also, the Japanese Laid-Open Patent Applications Nos. 39139/1978, 26139/1978, 5636/1978, 11036/1978, etc. have proposed to add the following heat-meltable substances of a lower melting point as sensitizers or melting-point-depression-agents: various waxes, fatty acid amides, alkylated biphenyls, substituted biphenylalkanes, cumarin compounds, diphenylamines, etc.
In the method for addition of sensitizer, it is required to melt the sensitizer prior to the color-forming reaction, so that the thermal response for a slight amount of heat energy in short time-pulse cannot be obtained satisfactorily in the high speed dynamic recording. The following problems occur due to the liquidation of the meltable substance in the color-forming layer: adhesion of residues onto the thermal head, bleeding, smearing, ghost, etc. In this case, under storage conditions at high temperature and at high humidity, the coloring of the background occurs with the time elapsed and the contrast of the recording image may be often degraded.
In accordance with the heating and the cooling cycle of the thermal head in the thermal recording process, the color-forming materials contained in the color-forming layer of a heat-sensitive recording sheet are once heat-melted and solidified, wherein a portion of the color-forming materials adhere to the surface of the thermal head, and consequently the melted heat-sensitive materials may be accumulated on the head surface as "residues", so that the quality of the recorded image is lowered.
Furthermore, if the thermal head adheres or sticks to the heat-sensitive sheet, that is, if a so-called "sticking" occurs, the movement of the sheet or the head will be obstructed with generation of offensive sounds and, in the worst case, the recording function itself may become impossible.
In order to resolve the above-mentioned problems, the Japanese Laid-Open Patent Applications Nos. 2793/1972, 33832/1973, 30539/1975, 145228/1977, 118846/1979, 118847/1979, etc. disclose the following methods: (1) the addition of filler such as clays, kaolin, talc, calcium carbonate, titanium dioxide, magnesium hydroxide, magnesium carbonate, etc. into the color-forming layer, (2) the increased addition of binder into the color-developing layer, or (3) the addition of waxes, releasing agent, starch particles, pulp-powder, coarse inorganic pigments, etc., into the color-forming layer. But these methods do not produce sufficient effects, and cause the increased amount of coating, the lowering of image density and the increased adhesiveness to the thermal head. Also sufficient results are not always obtained by these methods.